My only real frustration when using Linux is a fault that seems only to happen on some boxes and not others, and only when using GLX/3D acceleration, especially on nVidia but also occasionally on ATi.

What happens is that everything freezes except the mouse pointer which moves around the screen but its cursor stays fixed. Apparently if the computer is on a network, you can SSH into it (whatever that means) and find that X has taken over all your memory, you can close X and the computer is still running. Otherwise you are forced to press ALT-SYSRQ-S to save all open files, ALT-SYSRQ-U to unmount them and reopen them as read-only, and ALT-SYSRQ-B to reboot. There are other combinations that restart the keyboard, etc, but ultimately you never get up and running properly unless you reboot.

There are several theories about the origin of this fault. One is the nVidia drivers. I'm not sure about this, because I have read that it can happen in any 3D-enabled X setup. Another is the kernel. Another is somewhere in X.

Everyone blames the other, and you only read about it on forums. It never seems to be discussed in magazines, or acknowledged by any developers. At the nvidia help site, it is one of the biggest topics, but no "official" answers at all.

It also seems to strike randomly, I didn't have this problem for ages, and then it got so bad that I had to switch to the nv (2D) driver which is slow even in 2D.

It can strike in any distro, and there is no rhyme or reason. Two people with identical computers and identical distros can find that one persion never sees this bug and another sees it all the time. A little tweak of xorg.conf can result in it going away for a while only for it to come back with a vengeance later.

If developers can't work out what's causing it, can they at least add a means of escaping from it without having to reboot? Has anyone else had this problem?

It happened to me yesterday but I did not know the keyboard sequence to reboot so had to resort to reset button which then set off whole sequence of fsck stuff but the system did eventually get going again!
so thank davecs for the tip will bear it mind if it happens again
Dick

One thing to be aware of with the alt+sysrq sequence, some distros disable this function by default for security reasons. SuSE, for example: one of my first tasks after a fresh installation of a SuSE distribution is to re-enable it.

There is a full explanantion of all the magic sysrq key sequnces in the kernel docs, such as
/usr/src/linux/Documentation/sysrq.txt
on SuSE. IIRC, it has also been dealt with briefly in the magazine.

If you don't actually need 3D/GLX, the "nv" solution is OK, it just slows the machine down so much. Oh and if you have an analogue connection to your monitor, it shifts the picture over, about 6-7%, so you have to adjust it, which is annoying if you need to change back and forth.

But still, is there a way to force this issue right out into the open so that the developers will get their heads together and deal with it? Maybe we need a "bribery" fund for them.

The problem is that whilst X is running, it controls the keyboard. It is X that has locked up, not the kernel. The only keys that work at this point are the ALT-SYSRQ-key combinations, because they are directly controlled by the kernel!

So the problem is definitely X. But a lot makes up X, it's a question of which part and who is responsible. Or is a clash with part of the kernel to blame?

It doesn't sound like a kernel issue. If, as you said earlier, you can still SSH into the computer from another, the OS is running fine. The problem is that X has locked up and, as you say, X is grabbing almost all input so you cannot get out of it. Using SSH bypasses all of that since it sets up a new console login that has nothing to do with X. From there you can kill X.

A kernel level lockup would also kill networking, so SSH wouldn't work.

Last edited by nelz on Sat May 14, 2005 2:15 pm, edited 1 time in total.

Why? You should have been able to switch to another terminal and shutdown gracefully. That is the purpose of alt+sysrq+r, to free the keyboard from a locked X session and allow you to switch to a terminal where you can do things in a more controlled manner.